Sargeras is of course more powerful. Archimonde more directly destructive. But for sheer malice, for spite, for a turn of mind so devious and sinister that it pursues vengeance for over 25,000 years for the slight of not wanting to become a monster, you can never find evil more cunning and persistent than that of Kil'jaeden the Deceiver.
Make no mistake. Even Gul'dan pales in comparison to the hate, greed, and wanton cruelty that motivates Kil'jaeden. The demon lord par excellence, when we look at many of the greatest ills of the modern age it was the hand of the Deceiver that shaped them. The corruption of the orcs? The creation of the Scourge and the Lich King? The Third War? Others may have taken these actions, pursued them to their ultimate conclusion, but it was the mind of the Deceiver that brought them forth and worked to make them a reality. Indeed, it's fair to say that Kil'jaeden is often far more successful when he can resist taking an active hand in events - his most recent defeat at the Sunwell took place because he chose to attempt what Sargeras had failed to do and what had killed Archimonde, namely the bodily invasion of Azeroth proper.
So let us look now upon the Deceiver, lord of lies, spreader of falsehood - liar and betrayer of betrayers.